ABSTRACT Heparin-induced thrombocytopenia (HIT) is a potentially devastating prothrombotic complication of heparin therapy. Great strides in understanding the pathobiology of HIT have been made by our group and others using in vitro and murine models of disease. However, translation of this work to humans has been hampered, in part, by a paucity of well-annotated clinical samples and lack of a universal standard for diagnosis. We hope to overcome these limitations and facilitate extension of the individual projects in this Program Project to human disease by establishing a clinical registry and repository in part based on a validated clinical scoring system for HIT developed by our group. Service 1: Establishment of a clinical registry. Adult patients at the University of Pennsylvania (UPENN), Duke University Medical Center (DUMC), and Thomas Jefferson University (TJU) in whom a diagnosis of HIT is suspected will be offered enrollment in the registry: Our scoring model will be applied prospectively to each subject to vigorously and systematically characterize disease status. Additional clinical and demographic variables of interest will also be collected. Service 2: Establishment of a repository. Patients who enroll in the registry will also be asked to provide an acute blood sample for plasma. In addition, DUMC will provide samples from patients with a likely diagnosis of HIT, who have undergone treatment with plasmapheresis. An aliquot of plasma from each subject will be assessed for HIT antibodies in the laboratory of Dr. Cines using an anti-platelet factor 4/heparin (PF4/H) ELISA and a functional serotonin release assay. Service 3: Studies of HIT pathobiology in humans. Availability of carefully annotated biological material from HIT patients based on rigorous prospective clinical assessment will be used to extend the proposed murine-based experiments to confirmatory studies with isolated HIT plasrna and, in conjunction with Core B, isolated HIT IgGs and affinity-purified anti-PF4/H IgGs. Project 1 will utilize affinity-purified batched/standardized HIT IgGs to confirm in vitro and in vivo murine HIT studies conducted using KKO on the role(s) of platelets, monocytes and the endothelial lining in the development of thrombocytopenia/thrombosis. Project 2 will utilize unbatched HIT IgGs to confirm the PF4 epitopes and antigen-antibody interactions defined using KKO, RTO and novel murine anti-PF4/H monoclonal antibodies. Similar studies will be performed in Project 3 using antibodies developed in the active murine HIT model. The efficacy of novel therapeutics in Project 4 will also be tested in the presence of HIT IgG as well as with KKO. Finally, remaining annotated and either unbatched or batched material, not being entirely utilized within the Program Project, will be shared with other investigators in the field. Thus, Core C will provide critical clinical maternal to facilitate confirmation of the mechanistic and therapeutic studies conducted in the various Projects, thereby enhancing the clinical applicability of the findings emerging from this Program Project.